In typical gas discharge lasers, the performance of the laser can depend, at least in part, on the temperature of the laser gas in the discharge chamber. In many existing gas discharge laser designs, the gas circulation fan and laser discharge are utilized as the primary sources of heat for the laser tube. Using such a design, however, results in an undesirably long time to sufficiently heat the laser tube, or discharge chamber, especially after filling the chamber with a fresh supply of laser gas. Further, the temperature in the laser tube will vary over time due to the burst operation of such a laser. These temperature variations can lead to a corresponding yet undesirable variation in laser output.
Conventional approaches to regulate temperature for gas discharge lasers, and minimize temperature variations, typically involve cooling the laser gas during laser operation, such as by using cooling water flowing through a cooling loop. The loop generally includes tubing exposed to the gas mixture such that the flowing water can remove heat from the mixture. This heat is introduced into the gas mixture during operation when discharges of electrical power are applied to the gas mixture, through main discharge electrodes and/or preionization electrodes. Even with the cooling water flowing and removing heat, the gas mixture typically remains far above room temperature while the electrical pulses are being applied. If the laser pulsing is interrupted, the flow of cooling water can be halted or closed off. Regardless, the laser gas tends to cool down as there is little to no heat added when the laser is not pulsing. When the pulsing is resumed, the gas mixture will heat back up, such as to a cooling-water-stabilized value, within some initial period. While the gas mixture is heating during this initial period, the laser beam parameters can be influenced by the changing temperature.
FIG. 1(a) schematically illustrates a side view of a gas discharge laser tube 100 including a heat exchange circuit 102. The gas discharge laser tube 100 is filled with a gas mixture. A pair of main discharge electrodes 104, as well as one or more preionization electrodes 106, is connected to an electrical discharge circuit (not shown) for energizing the gas mixture as is known in the art. A discharge region 114 is defined between the main electrodes 104. The laser tube has a pair of windows 108 for permitting generated light to exit and enter the tube within the laser resonator (not shown). The heat exchange circuit 102 includes tubing 110 that enters and exits the laser tube 100, allowing fluid to flow into and out of the laser tube. The fluid carries heat away as the fluid exits the laser tube. A fan 112 in the tube circulates the gas mixture through the discharge region 114 past the heat exchange tubing 110, aiding in heat removal.
FIG. 1(b) schematically illustrates a cross-sectional view of the laser tube of FIG. 1(a), including electrodes 104, 106 that define the discharge region 114, the windows 108, the heat exchange tubing 102, and the blower 112, as mentioned above. An upstream gas flow direction 116 and a downstream gas flow direction 118 are depicted. A temperature sensor 120 is also shown, which is disposed downstream of the discharge region 114 and above the heat exchanger 102, such as is described in U.S. Pat. No. 6,034,978, which is hereby incorporated herein by reference.
FIG. 2(a) schematically illustrates a conventional heat exchange circuit 200 for a gas discharge laser, such as an excimer laser. A discharge circuit 202 is shown adjacent to the laser tube, or discharge chamber 204. Inlet tubing 206 carries fluid into the discharge chamber 204 at temperature Tin, while outlet tubing 208 carries fluid out of the chamber at temperature Tout. A control valve 210 can open and close a fluid flow loop created between the discharge chamber and an external fluid source 212. A temperature sensor 214 can be used to measure the gas temperature TG within the chamber 204. A flow control module 216 can be used to adjust the control valve 210 based on the measured temperature TG.
FIG. 2(b) illustrates the heat exchange circuit of FIG. 2(a) including a heater 218 positioned between the external fluid source 212 and the discharge chamber 204. The fluid flows past the control valve 210 to the heater 218 at temperature Tin1. The fluid is heated by the heater 218, such that the fluid flows from the heater to the discharge chamber 204 at an increased temperature Tin2. The controller 216 can control both the control valve 210 and the heater 218 based at least in part on signals received from the temperature sensor 214. Cooling water can be heated by passing the water through the heater 218 when burst operation of the laser is interrupted. Adding heat to the cooling circuit is, however, an inefficient and slow approach to altering the temperature of the cooling water during laser operation. Such an approach slowly increases the input water temperature, which flows through the heat exchanger within the laser tube.
When high duty cycle operation is restarted following a pause in high duty cycle operation for a long or short duration, the gas mixture temperature will slowly ramp back up to optimum temperature, which can disturb laser performance during the ramp-up period. Therefore, heat is added to the heat exchange fluid or directly into the laser chamber during the very low duty cycle or standby operation, or during a limited laser off time, to thermally stabilize the laser gas around optimum temperature. Conventional systems have used additional electrical heating of the gas in the laser chamber (see U.S. Pat. No. 5,377,215, hereby incorporated herein by reference) or by heating the incoming fluid in the heat exchanger tubing up to Tin2=40° C. . . . 60° C. (see U.S. Pat. No. 6,034,978, hereby incorporated herein by reference). Each of these methods requires an additional module, and is inefficient due to the initial low temperature which must be heated to a high temperature only when the high temperature is desired, thereby leaving an unsatisfactory delay and period of change from low to high temperature. In addition, a very high electrical current is used to provide the power to raise the temperature of the fluid and/or gas mixture to optimum, which is not practical. Moreover, gas temperature overshoots can occur, such as when energy from the heater cannot be instantaneously stopped at the beginning of high duty cycle operation and/or due to the inherently imprecise nature of warming and cooling a heating element across a wide temperature range. Further, these methods are very slow and are unable to adequately follow rapid changes in the mode of laser operation, such as between very low (idle) and very high (exposure) duty cycles of laser operation, with cycle times between fractions of seconds and several minutes.
A gas temperature deviating from the optimum can cause substantial distortions in laser parameters such as pulse-to-pulse stability, overshoot control and burst behavior in general that are unsatisfactory. Even temperature fluctuations of a few degrees centigrade from optimal can have a negative impact on the performance of the laser system.